Field Of The Invention
The present invention falls within the technical field of the preparation of silicone compositions. More specifically, the present invention relates to curable silicone compositions comprising a novel system for inhibiting hydrosilylation reactions.
Description of Related Art
In the field of silicones, hydrosilylation, also known as polyaddition, is a major reaction.
During a hydrosilylation reaction, a compound comprising at least one unsaturation reacts with a compound comprising at least one hydrogen atom bonded to a silicon atom. This reaction may be described, for example, by reaction equation (1) in the case of an unsaturation of alkene type:
or by reaction equation (2) in the case of an unsaturation of alkyne type:

Hydrosilylation thus makes it possible, firstly, to gain access to functionalized silanes or siloxanes, and, secondly, to form silicone networks by crosslinking between polysiloxanes comprising Si—H units and polysiloxanes comprising Si-vinyl units. A conventional use of this reaction is the formation of non-stick silicone coatings or films on paper or polymer supports.
Hydrosilylation reactions are conventionally performed by catalysis. Typically, the suitable catalyst for this reaction is a platinum catalyst, for instance chloroplatinic acid hexahydrate or the Karstedt catalyst which is constituted of platinum complexes with divinyltetramethyldisiloxane as ligand (see, for example, in U.S. Pat. No. 3,775,452). In small amount, this catalyst makes it possible to perform hydrosilylation reactions at room temperature with rapid kinetics, of the order of a few minutes.
In order to have the time to prepare, transport and use the composition before it has cured, it is often necessary to temporarily inhibit the hydrosilylation reaction. For example, when it is desired to coat a paper or polymer substrate with a non-stick silicone coating, the silicone composition is formulated to form a bath that must remain liquid at room temperature for several hours before being deposited on the substrate. It is only after this deposition that it is desired for curing by hydrosilylation to take place. Hydrosilylation reaction inhibitors may be of several kinds. The most widespread are thermal inhibitors. When maintained at room temperature, they inhibit the hydrosilylation reaction. By raising the temperature of the reaction medium, the inhibition is deactivated and the hydrosilylation reaction is activated. Conventionally, thermal activation is performed by introducing the substrates coated with the curable silicone composition into coating ovens whose temperature is maintained between 100° C. and 150° C. Examples of thermal inhibitors and of their use are described in patent applications WO 2011/076710, WO 2012/085364 and WO 2012/175825. The major drawback of the thermal activation of hydrosilylation is that it cannot be used on a substrate that is not heat-resistant.
To solve this problem, it has been proposed to use hydrosilylation reactions that can be triggered by exposure to UV radiation. To do this, one solution consists in providing a specific hydrosilylation catalyst that is activatable by UV radiation, as described, for example, in international patent application WO 92/10529. Another solution consists in using a standard hydrosilylation catalyst and in adding a photoinhibitor thereto: the function of the photoinhibitor is to prevent the hydrosilylation reaction when it is present in the reaction medium. However, in contrast with the thermal inhibitor, a photoinhibitor is not deactivated by heat, but rather by exposure to UV radiation.
Examples of photoinhibitors have been given in the literature. U.S. Pat. Nos. 4,640,939 and 4,670,531 describe the use of azo compounds as reaction inhibitor. European patent application EP 0 238 033 discloses photocurable polyorganosiloxane compositions containing an essentially linear diorganopolysiloxane comprising chain-end vinyl functions, an organohydrogenopolysiloxane, a platinum-based catalyst, a photosensitizer and optionally a modulating compound whose function is to prevent premature reaction of the curable compositions. The modulators described are acetylenic compounds such as 3,5-dimethyl-1-hexyn-3-ol, 3-methyl-1-butyn-3-ol, 3-methyl-3-penten-1-yne, 3-methyl-1-pentyn-3-ol and 5,5-dimethyl-3-hexen-1-yne. Moreover, U.S. Pat. No. 5,082,871 describes the use of compounds of acetylene-dialkyl dicarboxylate type as platinum-catalyzed hydrosilylation reaction inhibitor that is deactivatable by UV irradiation.
One of the essential functions of an inhibition system is thus to efficiently prevent the hydrosilylation reaction for as long as necessary before activation. To do this, it may be necessary to use large amounts of inhibitor, which causes strong inhibition of the hydrosilylation catalyst. The consequence of this is that the rate of curing of the composition, even after activation, is slowed down, which is a major drawback from an industrial viewpoint since this especially makes it obligatory to reduce the coating speed and thus the rate of production.
It would therefore be advantageous to have available novel hydrosilylation inhibition systems that are deactivatable thermally or by UV irradiation. It is desired to have available hydrosilylation-curable silicone compositions containing an inhibition system that simultaneously makes it possible:                to inhibit the hydrosilylation reaction for as long as necessary before activation,        to ensure rapid removal of the inhibition at the time of activation, and        preferably, to ensure a high rate of crosslinking.        